Need input on my TL. FR has a nasty dip in it.

I've been testing my newly constructed TL today with the help of Pink Floyd and test tones. What i'm noticing is a dip in output from 61-85hz. At it's center, around 75hz, there is very little sound at all. Otherwise, as compared to a sealed cabinet I have for this sub (.95qtc), 40-60hz is definantly stronger, and below that is approximately the same or slightly better on the TL (at least to my ears). It's hard to make determinations between small differences when it takes so long to transplant the driver from one cabinet to another.

Stuffing helped with the overall sound. I ended up using way more then I was expecting to. But didn't have much effect on the void. I'm about to test if line length effects it. At the moment i'm using a passive 80hz xover. Which I think is a bit high. Soon as I get my amp back, I'm thinking 65-70hz will be a better xover.

Here's my setup...

Sub is placed about 5ft behind listening spot. Speakers are about 9ft in front.

25.5hz
133.1l vas
4.16ohm
.28qms
4.57qes
13.2Bl
510cm sd

The TL is 101" long right now. The finished line will probably be 115". Inner diameter is 12.5".

Any input or ideas would be greatly appreciated. I just hope I hadn't chosen a bad project for my first one. From my readings here, and with MJK's spreadsheets, it seemed easy enough. :)